The invention relates to novel carbonate containing mineral fillers, more particularly natural and/or precipitated calcium carbonates, which may advantageously be employed as matting agents, more especially matting agents for aqueous coating systems.
Dispersion paints are coating materials, which are utilized for the protection and optical embellishment of building structures. Interior dispersion paints for use in buildings protected against the elements constitute the most important and largest group in this respect.
Frequently a dull to, at most, silk sheen impression is desired for coated walls. The aim of providing such impression is something having to be fulfilled by paints. In order to ensure that paints possess minimum gloss, matting agents must be added to the paints, which suppress directional reflection of light to a very substantial extent.
Matting agents are classified as fillers and are characterized by special grain properties, which contribute to a reduction in gloss of a paint surface. Gloss reduction is caused by the systematic creation of finest-roughness (micro-roughness) of the paint surface so that the dispersion of light is increased and directional reflection is reduced.
Micro-roughness may be produced in various ways: in the case of paints with a continuous binding agent skin on the surface it is caused by incorporating a suitable quantity of particles which are coarser than the rest of the components and which then give rise to a homogeneous "hill landscape". In this respect attention is to be paid to the resulting overall grain size distribution of all solid components. If there is an insufficient coarse fraction, there is an "island effect" which is characterized by individual grains projecting like islands from a homogeneous, smooth film. With the naked eye it is then possible to detect such grains as low-gloss structures in the shining overall film.
Super-critical paints, whose binding agent fraction is insufficient to encase all solid components completely and additionally to fill up the narrowing cusp-like gaps between the individual grains, offer another possibility for reducing gloss. Owing to the lack of binding agent the dry paint film contains air pores. Such pores, which result in openings at the surface, cause surface discontinuities and consequently roughness. Accordingly the gloss level of super-critical paints is generally lower than is the case with sub-critical ones. If a very porous filler (with a high internal surface) is added, such filler will take up binding agent in its pores, accordingly increasingly the formation of air pores in the paint film and hence lowering the degree of gloss. If such porous filler (for example diatomaceous earth) is coarser than the remaining components, there will be additionally the matting effect described in the case of sub-critical paints owing to there being a change in grain (.fwdarw."hill landscape formation").
In the literature material is to be found on matting high-gloss paints to a medium gloss level (Huemmer T. F., Wasowski L. A., Plooy R. J.: "Pigmentation of radiation curable systems", "Journal of Paint Technology" vol. 44, No. 572, September 1972, pages 61 through 74).
There is no literature on the subject of additional matting in the semi-matt to matt range, that is to say paints which already have a very low initial gloss level. Matting agents are not only employed for dispersion paints but for all paints and varnishes whose degree of gloss is to be reduced. Examples are UV curing paints, polyurethane varnishes or also plastic coatings.
The most widely employed matting agent for all these applications and more particularly for dispersion paints, consists of more or less pure silicon dioxide (SiO.sub.2). It is extracted from natural deposits or is produced synthetically. As a natural silicon dioxide for matting purposes its porous form is employed, which is known as diatomaceous earth, diatomite, siliceous earth or kieselguhr. In this respect it is a question of a white, yellowish or gray friable mineral, which is very porous, light and fine grained. Mineralogically such material contains opal as its main mineral, in addition to which, dependent on its degree of purity, it either contains no further components or contains clay minerals and/or other silicates. Such diatomaceous earth is employed as a matting agent either directly or after calcination. The characteristic features are a high porosity and specific surface as well as a low bulk density.
In addition to silicon oxide hollow polymer spheres with a grained surface, lamellar silicates such as mica and talcum and cellulose fibers are also employed as matting agents. While hollow polymer spheres have no significance as matting agents for dispersion paints and lamellar silicates are merely utilized as parts of components, which by themselves do not possess any potent matting action, by using cellulose fibers an effective reduction in gloss may be produced. Alongside silicon oxide products they constitute the most important group of matting agents for dispersion paints. For the production of the cellulose fibers wood is freed of lignin and hemicellulose after disintegration by chemical digestion and then bleached with chlorine. The chips are defibrillated after this and then fractionated and dried.
Both silicon oxide products and also cellulose matting agents are objectionable from the environmental point of view.
Independently of their crystal structure, silicon oxide products present a substantial direct health hazard for humans owing to the risk of silicosis. Silicon oxide dust, whose grain size is below 10 .mu.m, penetrates into the lungs where it causes tissue hypertrophy. It is known that acute silicosis, which may occur even after short exposure to dust containing high concentrations of silicon oxide containing dust (&gt;20% SiO.sub.2), as a rule quickly leads to alterations in the lungs and normally to early death. The long term action of small doses may frequently mean that there is a latent state lasting decades which, even after discontinuance of the exposure to dust, will still be responsible for symptoms such as a proneness to fatigue, loss of appetite, loss of weight, coughing and increasing breathlessness.
Persons having to work with silicon oxide products (as for example in manufacturing or processing operations) are subject to a high hazard. The hazard particularly applies for persons mechanically working on materials containing silicon oxide, as for example during abrasive stripping of old dispersion paints. In Germany silicosis is a notifiable industrial disease and is subject to compensation.
Cellulose fibers, which are utilized as a matting agent, must possess a high degree of whiteness in order not to compare unfavorably with silicon dioxide products having a brightness of 90 to 93%.
Cellulose with such a brightness may only be produced by an intensive and chemically aggressive bleach. The only element that causes such a powerful bleaching action is chlorine. Therefore cellulose is bleached with gaseous of dissolved chlorine--a method which in the paper industry was under considerable attack even some years ago. In fact during bleaching aromatic halides (AOX) are formed, which are classified as being carcinogenic. While for cellulose, which is employed in the paper making industry, chlorine-free bleaching methods have been developed, in the case of matting agent cellulose it is not possible presently to dispense with a chlorine bleach because of the substantially higher standard of whiteness required in accordance with the present state of the art. While alternative bleaching methods (for example ozone bleaching) are being tried out, they presently yield cellulose with a substantially lesser degree of whiteness whose use would affect the brightness of the coat of paint in an undesired fashion.
The German industrial standard (DIN) 53 778 ("Kunststoffdispersionfarbe fur Innen" (plastic dispersion paint for interior use) DIN 53 778, parts 1 through 4 (August 1983) sets the following minimum requirements for paints:
Working must be possible, using commercially available equipment, by brushing, rolling or spraying. In order to meet this requirement a suitable paint viscosity and in accordance with practical requirement for airless spraying a limitation of the maximum solids grain size to 60 .mu.m are required.
Dispersion paints must be able to be diluted with water without separating into different phases and must be compatible with toning colors.
In the case of a minimum working temperature of +5.degree. C. the dispersion paints must still able to be applied and able to form a film, if no other minimum processing temperature is specified by the manufacturer.
Carbonate containing mineral fillers, more particularly natural and/or precipitated calcium carbonates, have already been known in the art for some time. It is only by way of example that reference should be had to the German patent publication 4,128,570 A1, which discloses carbonate containing mineral fillers, pigments or similar materials, which are characterized by a mean statistical particle diameter of 0.4 to 1.5 .mu.m.
Such materials are unsuitable as matting agents for aqueous paint systems because they are much too fine and therefore cannot develop a matting effect.
In view of the above the following basic consideration arises: Obviously the coarser its grain distribution, the more effective a matting agent is. On the other hand a commercial product must be able to be readily processed; only by way of example it is necessary to keep to a limit for the maximum grain size of the solids of 60 .mu.m. Independently of this the disadvantages of the above discussed prior art matting agents should be avoided.